The present invention relates to a charging apparatus for charging an object. More precisely, it relates to a contact type charging apparatus (contact charging apparatus) which charges the surface of an object by placing a charging member in contact with the object.
The present invention also relates to an image recording apparatus (image forming apparatus), such as a copying machine, a printer, or the like, which employs a charging apparatus of the above described type as a means for charging an image bearing member of the image recording apparatus.
In a conventional contact charging apparatus, the surface of an object to be charged, for example, the surface of an image bearing member of an image forming apparatus, is charged to predetermined polarity and potential level by applying a predetermined charge bias to an electrically conductive member (contact type charging member, contact charging device) in the form of a roller (charge roller), a fur brush, a magnetic brush, a blade, or the like, while keeping the electrically conductive member in contact with the object to be charged.
All charging apparatuses which match the above description regarding a contact charging apparatus are roughly categorized as a contact charging apparatus. However, there are large differences in terms of charging mechanism (or charging principle) among the contact charging apparatuses. There are two types of contact charging mechanisms: (1) the charging mechanism based on electrical discharge, and (2) the charging mechanism based on direct charge injection. Which of the two types of charging mechanism is employed by a contact charging apparatus determines the characteristics of the contact charging apparatus. Thus, the charging principles and characteristics of the electrical discharge type charging mechanism and direction injection type charging mechanism will be described next.
(1) Charging Mechanism Based on Electrical Discharge
This is a charging mechanism which charges an object with the use of the products generated by the electrical discharge which occurs through the gap between a contact type charging member and the object to be charged.
An electrical discharge type charging system is characterized by an electrical discharge threshold value: there must be a certain amount of voltage difference between a contact type charging member and an object to be charged. More specifically, referring to FIG. 7, a voltage, the level of which is higher than the potential level of the object to be charged, needs to be applied to the contact type charging member, as is represented by Line A in FIG. 7. Further, in principle, it generates by-products, admitting that the amount of the by-products is drastically smaller than that generated by a corona type charging apparatus.
From the standpoint of safety regarding electrical discharge, a roller based charging method (roller charging apparatus), which employs an electrically conductive roller (charge roller) as an electrical discharge based contact type charging member is preferable and is widely in use. An electrical discharge type charge roller comprises a base layer and a surface layer. The base layer is in the form of a roller and is formed of rubber or foamed substance, which is electrically conductive, or the electrical resistance of which is in the medium range. The surface layer is high in electrical resistance and covers the base layer. Electrical discharge occurs through the gaps between the roller and the object to be charged, immediately adjacent to the contact (interface) between the roller and the object, which are several tens of micrometers wide. Therefore, in order to stabilize the electrical discharge, the outward surface of the surface layer is rendered very smooth; its roughness in terms of Ra is less than one micrometer. Further, it is rendered high in hardness.
In order to charge an object with the use of an electrical discharge type roller, the voltage to be applied to the roller must be relatively high. Therefore, if there is a pinhole (exposure of the base layer of the object to be charged, caused by the damage to the surface layer of the object) in the surface of the object to be charged, not only does voltage drop occurs at the point of the pinhole, but also in the immediate areas of the pinhole, causing the object to be charged to be improperly charged. Thus, in order to prevent the voltage drop, the surface resistance of the surface layer is made to be no less than 1011 ohmxe2x96xa1.
(2) Direct Injection on Type Charging Mechanism
The direct injection type charging mechanism is a charging mechanism for charging the surface of an object to be charged, by directly injecting electrical charge into the object to be charged, from a contact type charging member, through the molecular level contact between the contact type charging member and the object to be charged. It is sometimes called a direct charging mechanism or an injection charging mechanism.
In this charging mechanism, that is, a direct injection type charging mechanism, the difference in potential level between a contact type charging member and an object to be charged is no more than several volts to several tens of volts. Line B in FIG. 7 represents the charging performance characteristic of a direct injection type charging mechanism (magnetic brush type charging apparatus). In this case, the potential level of the voltage applied to a charging member is equal to the potential level to which an object will be charged. Therefore, there is no difference in potential level between the charging member and the object to be charged, which causes electrical discharge. Also in this case, the voltage necessary for charging an object can be kept low in potential level.
As described above, a direct charging system as a charging mechanism is not accompanied by ion production, and therefore, it does not cause problems related to the byproducts of electrical discharge. In other words, a direct charging system is a superior charging system in terms of environmental safety, component deterioration, and power consumption.
Next, a charging apparatus employing a direct injection type charging mechanism will be described.
In a direct injection type charging mechanism, one of the essential factors which determine the charging performance of a direct injection type charging mechanism is the state of contact between a contact type charging member and an object to be charged. This state of contact means the number of contacts, in microscopic terms, the contact type charging member makes contact with the object to be charged, while the object is passing through a charging apparatus. Thus, it is required that not only does the contact type charging member have an extremely fine surface structure, but also it must have a sufficient amount of elasticity for keeping its surface in contact with the surface of the object.
As for the configuration of a contact type charging member employed by a direct injection type charging apparatus, an electrical discharge based charge roller or the like has been widely tried. However, the attempts to use an electrical discharge based charge roller for direct charge injection have been unsuccessful. This is due to the following fact. That is, in the case of a charge roller which has a hard and smooth surface, its surface seems to be perfectly in contact with the surface of an object to be charged, but, at a molecular level, that is, at a microscopic level, there is virtually no contact between the surfaces of the charge roller and the object to be charged.
As a presently proposed direct injection type charging mechanism, there is a particle based charging method which employs a magnetic brush.
(3) Particle Based Charging Method
From the standpoint of improvement in contact density, a charging method which employs electrically conductive particles (particle based charging method) is advantageous. The electrically conductive particle used for a particle based charging method hereinafter will be called xe2x80x9ccharging particlexe2x80x9d. As for an example of a charging particle, an electrically conductive magnetic particle may be listed as a typical one, and there have been made several proposals in which a magnetic brush type charging member is formed with the use of electrically conductive magnetic particles and a magnet.
FIG. 8 is a sectional view of a magnetic brush type charging apparatus 100, for showing the general structure thereof. A referential code 120 designates a magnetic brush type charging member, which comprises: a stationarily supported magnetic roll 122; a nonmagnetic and electrically conductive charge sleeve 121, which is rotationally and coaxially fitted around the magnetic roll 122; and a magnetic brush layer 124 (magnetic brush portion) formed on the peripheral surface of the charge sleeve 121 by adhering and holding electrically conductive magnetic particles C to the peripheral surface of the charge sleeve 121, with the use of the magnetic force of the magnetic roll 122 within the charge sleeve 121. A referential code 123 designates a casing, to which the magnetic brush type charging member 120 is attached, and in which a proper amount of electrically conductive magnetic particles C is stored. A referential code 125 designates a magnetic brush layer thickness regulating blade, with which the casing 123 is provided.
A referential code 1 designates an object to be charged, which in this embodiment is an electrophotographic photoconductive drum and is rotationally driven in the clockwise direction indicated by an arrow mark. In this magnetic brush type charging apparatus, the magnetic brush layer 124 of the magnetic brush type charging member 120 is placed in contact with the photoconductive drum 1 as an object to be charged, so that the width, in terms of the circumferential direction of the photoconductive drum 1, of the interface between the magnetic brush layer 124 and photoconductive drum 1 becomes a predetermined width. A referential code n designates the interface (charge nip) between the magnetic brush layer 124 and photoconductive drum 1.
More concretely, the charging sleeve 121 is a nonmagnetic and electrically conductive sleeve, which is 1.2 xcexcm in average surface roughness Ra1 of its peripheral surface. 16 mm in external diameter, and approximately 220 mm in length.
The magnetic roll 122 is provided with four magnetic poles N1, N2, S1, and S2, which are 800 G in peak magnetic flux density at the surface of the charging sleeve in terms of the radius direction of the magnetic roll 122. It is stationarily supported so that the magnetic pole N1 opposes the photoconductive drum 1.
As the electrically conductive magnetic particles C as charging particles which form the magnetic brush layer), magnetic metallic particles, for example, ferrite particles or magnetite particles, or particles formed by agglutinating these magnetic metallic particles, or the like, are used. The magnetic metallic particles used as the charging particles are 1xc3x97106-109 ohm.cm in electrical resistance value, and 10-50 xcexcm in average diameter.
The charging sleeve 121 is rotationally driven in the clockwise direction indicated by the arrow mark, as is the photoconductive drum 1. The magnetic brush layer 124 is moved with the charging sleeve 121 in the clockwise direction. As it is moved, it is regulated in thickness by the blade 125, and the thickness-regulated portion of the magnetic brush layer 124 makes contact with, and rubs, the peripheral surface of the photoconductive drum 1, in a contact charging nip n. The charging particles in the portion of the magnetic brush layer 124 which has passed through the charging nip n, are returned to the electrically conductive magnetic particle bin within the casing 123, from which they are recirculated.
As a predetermined charge bias is applied to the charging sleeve 121 from a charge bias application power source S1 while the peripheral surface of the photoconductive drum 1 is rubbed by the magnetic brush layer 124, electrical charge is directly injected into the peripheral surface of the photoconductive drum 1, in the charging nip n. As a result, the peripheral surface of the photoconductive drum 1 is uniformly charged to predetermined polarity and potential level.
To note the contact density of the electrically conductive magnetic particles of the magnetic brush layer 124 in the above described structure, when the average external diameter of the electrically conductive magnetic particles is approximately 30 xcexcm, and the contact density is approximately 103 point/mm2, which provides good charging performance represented by the line B in FIG. 7.
Under the above described condition, the amount of the electrically conductive magnetic particles needs to be several hundreds of mg/cm2 , and those particles held to the peripheral surface of the charging sleeve 121 form a 0.5-1.0 mm thick particle layer. In charge injection, it is required for a contact type charging member to contact an object to be charged, elastically, and densely in terms of the number of contacts. However, in the case of the magnetic brush type charging member 120, electrically conductive magnetic particles must be magnetically held to the peripheral surface of the magnetic brush type charging member 120. Therefore, the charging sleeve 121 as a particle bearing member must be a rigid member. Further, the flexibility of the contact type charging member must be provided by the magnetic brush layer formed of electrically conductive magnetic particles. Therefore, the magnetic brush layer must be proper in thickness and also proper in the amount of particle per unit area of the peripheral surface of the magnetic brush type charging member 120.
A charging particle based charging method is suitable for a toner recycling system. In a toner recycling system, waste toner particles (transfer residual toner particles) in an image recording apparatus of a transfer type are recycled for image formation. Therefore, not only is the toner supply more efficiently used, but also the space for a cleaning means container can be eliminated, making it possible to reduce apparatus size. In other words, a toner recycling system is an excellent system.
More specifically, the transfer residual toner particles are rendered reusable by being taken into a contact type charging member (given original amount of electrical charge), and are returned to a developing apparatus by way of an image bearing member, to be used again for development, or to be recovered for recycling. Thus, a charging apparatus used with a toner recycling system must be enabled to recover the transfer residual toner particles and recharge toner particles, in addition to being enabled to charge an image bearing member.
From the above described standpoint, the compatibility of a magnetic brush with a toner recycling system will be examined. A magnetic brush is characterized in that it is formed of magnetic particles, being therefore enabled to easily move or conform to the shape of an object to be charged, and also in that it is relatively large in specific surface area. Therefore, the mandatory functions of a toner recycling system, for example, recovering the transfer residual toner particles from an image bearing member, or properly charging the transfer residual toner particles after the recovery, can be easily realized with the employment of a magnetic brush.
As is evident from the above description, a magnetic brush type charging apparatus is an excellent charging apparatus. However, a magnetic brush type charging apparatus is desired to be further improved in toner recycling performance. For that purpose, it is desired that the average diameter of the charging particles is further reduced so that a contact type charging member which is higher in contact density and greater in conformity than currently available ones.
However, in the case of a conventional magnetic brush based charging method, as electrically conductive magnetic particles are simply reduced in size (no more than 10 xcexcm), the effectiveness of the magnetic force, in terms of its ability to hold magnetic particles to a charge sleeve, reduces, creating a problem that magnetic particles fall out of the magnetic brush. The ill effects of such a problem are serious, since a magnetic brush type charging member is required to hold a certain amount of magnetic particles to its peripheral surface. More specifically, the magnetic particles which have fallen out of a magnetic brush cause such problems as adhering to an image bearing member, causing a latent image to be unsatisfactorily developed; transferring onto a recording paper, creating fog; or the like. In other words, they cause various image defects. As is evident from the above described problems of a conventional magnetic brush based charging method, simply replenishing a magnetic brush based charging member with magnetic particles is not a solution to the problems.
As described above, a conventional magnetic brush based charging method has a limit in terms of the improvement in contact density. Thus, the inventors of the present invention shifted their attention from the holding of charging particles with the use of magnetic force, and reviewed this problem from the viewpoint of the positive utilization of the attraction between two different substances.
As a result, it was discovered that it was important that the average size of charging particles was reduced, and that the amount of the charging particles held to the peripheral surface of the charging member was optimized. The discovery led to the development of a charging particle based charging apparatus, which is superior in charging performance to a conventional charging particle based charging apparatus, and yet does not suffer from the problems traceable to the particles which fall out of a magnetic brush. Incidentally, there have been known charging apparatuses employing electrically conductive nonmagnetic particles, some of which are disclosed in Japanese Laid-open patent Applications 10-307454-307459.
The primary object of the present invention is to provide a superior charging apparatus the superior charging performance of which is realized with the use of charging particles, and an image forming apparatus which employs such a charging apparatus.
Another object of the present invention is to provide a charging apparatus, which employs charging particles, and the contact density of which relative to an object to be charged is superior to that of a conventional charging apparatus, and an image forming apparatus which employs such a charging apparatus.
Another object of the present invention is to provide a charging apparatus, which is enabled to employ charging particles smaller in diameter than ordinary charging particles, and is structured so that the amount of charging particles on the charging particle bearing member is continuously optimized, and an image forming apparatus which employs such a charging apparatus.
Another object of the present invention is to provide a charging apparatus which does not adhere charging particles to an object to be charged, and an image forming apparatus which employs such a charging apparatus.
Another object of the present invention is to provide a charging apparatus, which does not cause image defects even if charging particles adhere to an image bearing member, and an image forming apparatus which employs such a charging apparatus.